Legumes are an important foodstuff and are likely to become more so in the future. Those in some of the legume genuses, such as Phaseolus and Soja, are high in protein and provide an inexpensive alternative to animal protein, for the most part lacking only in methionine to provide a balanced diet.
Unfortunately, ingestion of some of the most nutritious and abundant legume genuses can be accompanied by severe flatulence and associated abdominal distress. The effect on individuals varies widely, but with some people it is sufficiently severe that they avoid consumption of these foods entirely.
Quantitative research on the gas-producing properties of legumes commenced in about 1948. Subsequent research has involved work with many of the species in the genuses having the greatest gas-forming tendencies. In general, this work has been aimed at identifying those components of the seed that are responsible for flatulence. One of the significant investigations in this direction was reported by Steggerda, et al., in Soc. for Exper. Biol. and Med., 121, 1966, pp. 1235-9, who separated soybeans into various fractions and determined the flatus production in man with each of these fractions. They found that soybean hulls, fat, water-insoluble polysaccharides, and protein are not associated with flatulence production to any significant degree, and that the flatus-producing factor in soybeans was concentrated primarily in the low molecular weight carbohydrate fraction soluble in water. Research on other legume genuses pointed in the same direction. Compounds specifically identified as being a cause of flatus production are the alpha oligosaccharides, stachyose, raffinose, and verbascose.
The generally accepted explanation of the action of the alpha oligosaccharides in producing flatus is that the enzyme alpha galactosidase is not present in the intestinal tract of mammals. Thus, these compounds are not hydrolyzed and dissolved in the digestive tract so that they can be absorbed. Instead they reach the lower intestine essentially intact. Here anaerobic bacteria ferment these sugars with the resultant production of carbon dioxide and hydrogen, as well as some methane. Thus, these sugars are not only a source of discomfort but constitute a significant loss in nutritional value, since they amount to on the order of 4 percent of the total weight (dry basis) of white beans and 6 percent of soybeans.
Rackis in ACS Symposium Series 15, Sept. 11-12, 1974, pp. 207-221, reported an interesting experiment in which anaerobic cultures isolated from dog colon biopsies were used to treat a group of monosaccharides (glucose, maltose, fructose, galactose) and a group of oligosaccharides (sucrose, raffinose, and stachyose). It was found that the gas produced by all of these compounds (primarily carbon dioxide and hydrogen) was of essentially the same magnitude. Thus, any of these compounds reaching the lower intestine would result in gas formation. However, it is known that ingestion of either the monosaccharides or sucrose in normal quantities does not result in flatus. The significance of these results is that flatus production in the lower intestine may in part be related to solubility. The chemical structure of sucrose as a disaccharide is more similar to that of raffinose and stachyose than it is to the monosaccharides. Yet sucrose and the monosaccharides are highly soluble in water, whereas raffinose and stachyose have a much more limited solubility. High molecular weight polysaccharides, on the other hand, are so nearly insoluble that they may not be susceptible to anaerobic fermentation during the time of passage through the colon.